The mechanisms by which an individual generates and regulates a multitude of antibody-producing cells is being studied. The difficulty in this type of study is the enormous heterogeneity that exists within the potential antibody-producing cell repertoire. A small portion of this repertoire is being defined by immortalizing antibody-producing cells (specific for the hapten phthalate) by cell fusion analysis. Our efforts to define the secondary response to this hapten were initially frustrated by the apparent unlimited heterogeneity of this response (perhaps generated in part by random somatic mutations within the genes coding for the variable portions of the light and heavy immunoglobulin chains). Our recent efforts have focused upon the antibody-producing clones that appear very early (3 to 4 days) after a single immunization with a phthalate-protein conjugate. The rationale for this shift in focus was the possibility that fewer clones were stimulated early in the response and that these clones would represent conserved immunoglobulin gene products. Our findings are consistent with this assumption, and our endeavors have led to the generation of a hybridoma clone 2E9 that has had a significant positive impact upon our research program. The 2E9 hybrid clone secretes a phthalate-specific antibody whose idiotype appears repeatedly in cell fusion analysis of hybrid clones secreting anti-phthalate antibodies with either mu or gamma1 heavy chain isotypes. Analysis of the primary and secondary response sera of BALB/c mice (as well as several other strains) indicates that the 2E9 clonotype represents a major conserved phthalate-specific antibody-forming cell family. The significance of this finding to our program is that it enables us to monitor the effects of various perturbations upon the expression of this conserved clonotype. Earlier studies with a response of mice to dextran (a response that occurs without a requirement of T cells) demonstrated the significance of idiotype recognition in the regulation of expression of a dextran-specific clonotype. We are now able to determine if regulation of phthalate-specific clonotypes (a response requiring T cells) similarily involves idiotype recognition by regulatory cells. (LB)